Method of making magneticallyanisotropic permanent magnets



' METHOD OF MAG MAGNE'IICALLY- ANISOTROPIC PE s NENT MAGNETS No Drawing. Application December 10, 1952, Serial No. 325,202

Claims priority, application Netherlands December 21, 1951 15 Claims. (Cl. 252-625) This invention relates to magnetically-anisotropic permanent magnets and to methods of making the same.

The invention is particularly concerned with pennanent magnets made of materials of the type described in the copending United States patent application Serial No. 239,264, filed July 30, 1951. That application discloses a new class of permanent magnet materials composed principally of a composite oxide of iron and at least one of the metals selected from the group consisting of barium, strontium and lead. Such materials may be characterised by a hexagonal, for example, magnetoplumbite, crystal structure and have the composition MO.6Fe2O3 in which M is at least one of the metals of the above-mentioned group. Calcium may replace one of the metals in the crystals in an amount not exceeding 0.4 as an atomic fraction. In making such materials the starting materials are ferric oxide (FezOs) and one or more oxides of strontium, barium and lead, with the possible partial replacement of at least one of the latter oxides with calcium oxide. These starting'materials are mixed and heated to a temperature of about 900 C. to 1450" C., preferably between 1100 C. and 1450 C., for sutficient time to form non-cubic crystals consisting of material of the composition given above.

Such permanent magnets have numerous advantages, for example, the starting materials are readily available and inexpensive, the magnets can be manufactured in an inexpensive manner and the magnets have good magnetic properties, for instance, a field strength of dissappearance, I C of at least 700 Oersteds and a remanence Br of at least 1200 Gauss. However, the (BH)max Value of such magnets is not greater than about 1.1)( Gauss- Oersted per cubic centimeter which is below that of many of the known magnet steels.

The main object of the present invention is to materially improve the magnetic properties, particularly the (BH)max value, of permanent magnets of such materials.

A further object of the invention is to increase the (BH)max value of such permanent magnets up to 1.75 X10 and more.

A still further object is to produce a magneticallyanisotropic permanent magnet of excellent magnetic properties while using inexpensive and readily-available materials.

A further object of the invention is to provide inexpensive and simple methods of producing magneticallyanisotropic permanent magnets.

The permanent magnets according to the invention 7 comprise ferromagnetic material of non-cubic crystals, preferably of hexagonal structure, of a composite oxide of iron and at least one of the metals selected from the group consisting of Ba, Sr and Pb; the body being magnetically-anisotropic in a principal direction and having in said direction a (BH)max value which is more than 1.1, preferably more than 1.75, and which is between about 140% and 200% of anisotropic magnet of the same composition. The magnets are preferably sintered and an atomic fraction of not more than 0.4 of one or more of the metals Ba, Sr and Pb may be replaced in the crystals by calcium. In such magnets the ferro magnetic properties are determined substantially by single and/ or mixed crystals having the structure of magnetoplumbite of the composition MO.6Fe2Oa in which M indicates one or more of the metals Ba, Sr and Pb, with the possible partial replacement by Ca.

In accordance with the method of the present invention we produce the magnetic anisotropy by magnetically orienting the crystals, by mechanically orienting the crystals by altering the shape of the crystals mechanically, for instance, by pressure, centrifugal force or by rolling action at high temperatures or by combined mechanical and magnetic actions, while the crystals are arranged in a mobile condition, i. e. one which permits the necessary movement.

In one embodiment of the method of the invention We employ material of the type described above, which is in a finely-divided state, for example, particles less than 10y. and preferably less than 5;; and place the same in a sulficiently mobile condition so that the particles can be oriented by a magnetic field. The orientation is obtained by applying a magnetic field exceeding Oersteds, preferably about 700 Oersteds. The oriented material is then compressed into a compacted body, preferably while the magnetic field is still being applied. Instead of compressing the oriented particles, it is possible to sinter the same by heating to a temperature of about 900 C. to 1500 C., if desired, while maintaining the magnetic field.

In another embodiment of the method, the fine parti cles are put in a mobile condition by replacingthem loosely in a container which is vibrated while a magnetic field is applied to orient the particles. After the orientation the particles may be compressed or sintered into an adherent body with or without the presence of the magnetic field.

The ferromagnetic material may be reduced to the re-.

quired small particle size by mechanical means and many of the particles will be in the form of single crystals. As a result, the particles Will have decided preferential directions of magnetization which are parallel to a crystallographic main axis and which, in the case of hexagonal crystals, are parallel to the hexagonal crystal axes. As the particles will have some freedom of movement When in a mobile condition, they will, under the influence of the magnetizing field, tend to assume a position in which the main axis coincides substantially to the direction of the magnetizing field, i. e., the principal direction.

When the oriented particles are compressed, either during or after the application of the magnetic field and with or without the use of a binder, anisotropic perma-- ment magnets are obtained which may be finally magnetized. As a result of the compression, the oriented particles have little freedom of movement and become substantially fixed. Even if the oriented particles are not compressed until after the removal of the magnetic field, a large part of the relative parallel orientation of the magnetic preferential directions of the particles will still be maintained. However, we prefer to compress the particles while applying the magnetizing field as this improves the relative parallel orientation.

When heating the oriented particles, for instance, to the sintering temperature, disorientation from the principal direction may be greatly reduced by applying the magnetizing field until the Curie temperature has been reached. When not using a magnetic field there .is a possibility of disorientation, particularly at higher temperatures in the range up to the Curie point, due to the Patented Sept. 11, 1956 spite of the magnetizing field by closing the magnetic circuit with ferromagnetic material. However, the possibility of disorientation is reduced to a greater extent by using a magnetizing field.

We have found quite unexpectedly that when using the methods of the invention, even when the oriented material is not compressed, the orientation is to a large extent maintained when the bodies are heated to the sintering temperatures which are above the Curie point and above the temperatures at which crystal growth is liable to occur. In fact, the orientation will even be improved in well known fact that it is not possible to control the relative position of the particlesat such high temperatures by'a magnetic field. While the above is true when the bodies are not compressed, we prefer to compress the particles into a body while applying the magnetic field as this improves the (BI I)max "alue, even after the compressed particles have been sintered.

The orientation by a magnetic field may be effected at ordinary temperatures as well as at higher temperatures up to the vicinity of the Curie point. As a result, the

use of temperatures that do not exceed about 300 C. V

to 400 C., whether such temperatures are used intentionally or unintentionally,are not necessarily detrimental.

In another embodiment of the method of the iuven-;

tion, theparticles are placed in a mobile condition by distributing them in a. liquid dispersing agent, which preferably contains a suitable binder. If it is not desired to compress the particles into a body prior to the sintering,

the liquid may be removed, for example, by vaporization or suction and preferably while heating, in which case the binder holds the particles together and in proper orientation until the body is sintered. If the particles are to be compressed into a body, the liquid may be removed by compression with or without heating whereby a compacted body capable of being sintered may be obtained in a simple and inexpensive manner. In this case, the binder serves to further ensure that the particles will not become disorientated.

As a dispersing agent for holding the particles in a mobile condition, we prefer to use a liquid whose viscosity is so low as not to hamper the rotation of the preferential directions of the crystals into the direction of the magnetic field used during orientation at low temperatures. On the other hand, the liquid should be one in which, after the removal of a large part thereof, the residual binder will hold the particles together in a body which is sufiioiently strong that the magnetic forces produced therein by the demagnetization field will not cause the body to break. A suitable dispensing agent is polyvinylacetate. Water may also be used as a dispersing agent but, in the absence of a sufficient quantity of a binder, this has the limitation that, after the removal of the greater part of the Water, the particles are not held together sufiiciently, to prevent disorientation or breakage.

In another embodiment of the method the particles are placed in a metal shroud or closed tube,for instance, of-

stainless steel, which is rolled at a temperature of 800 C. or more, preferably between 1000 C. and 1200 C., to produce the orientation. This rolling process, which alters the shape of the crystals, may be followed by a sintering process. The particles used may be either the final materials or the starting materials described in the above-mentioned patent application and an external magnetic field may be applied during the rolling operation.

The permanent magnets according to the invention are finally magnetized in the principal direction, for instance after they have been sintered while the magnet body is being cooled down from the sintering temperature, particularly in the range of temperatures below the Curie point. Compressed magnets which are not to be sintered air. I grinding in a ball mill for 12 hours with alcohol.

may be finally magnetized by a magnetic field or by the magnetic field used to orient the crystals if this field is chosen sufiiciently high so as to obtain magnetic saturation of the material.

In order that the invention may be clearly understood and readily carried into effect we shall describe the same in more detail with reference to a number of examples.

Example 1 A mixture of 450 grams of barium carbonate (68.6% by weight of Ba) and 2000 grams of FezOa (68.4% by weight of Fe) was ground in a ball mill for hours under alcohol. After drying the powder obtained was passed at a rate of 20 mms./min. through an electrical furnace having .a heating zone 20 cms. long and maintained'at a temperature of 1100 C. The firing period was consequently 1.0 minutes and firing was effected in After being cooled, the material was pulverized by After being dried the prefired powder thus obtained was stirred with a16% solution of polyvinylacetate in acetone to formed was introduced form a suspension containing 2 cos. of the binder solution per gram of the powder. This suspension was placed in a brass die positioned between the poles of a magnetic yoke. The particles were subjected to a first field of 9500 Oersted while the acetone was removed by evapora- The small block of anisotropic material thus into an electric furnace while postioned between poles of a magnetic yoke. While a second field of 2000 Oersteds was provided in the preferential direction of magnetization determined by the first field, the temperature was increased to 500 C. The field was removed and the material was sintered by passing the same at a rate of 20 mms./min. through an electric furnace having an atmosphere of air at a tern perature of 1280" C. Since the heating zone was 10 cms.

in length, the smtering period was 5 minutes. The sintered body so obtained had a remanence of 2650 tion.

Gauss, a field strength of disappearance I C of 177-5 Oersteds and a B C of 1500 Oersteds in the principal direction. While the apparent density was only 3.9, a (BH)max value of 13x10 was obtained. At right angles to the principal direction, the remanence was 1260 Gauss.

Example 2 Pre-fired powder obtained in the manner described in Example 1 was stirred with a 8% solution of polyvinylacetate in acetone to form a suspension, use being made of 4 cos. of the binder solution per 3 grams of the powder. The suspension was placed in a molding die positioned in a first magnetic field which was comparatively weak having a value of 2000 Oersteds. After the magnetic field was removed and the binder solution was removed, the powder was compressed into a block. The remaining binder solution was removed by firing the block in a second magnetic field of 2000 Oersteds in a manner similar to that described in Example 1. When a temperature of 500 C. had been reached, the magnetic field was removed and the block was sintered by passing the same at a rate of 20 nuns/min. through the furnace which had an atmosphere of air at a temperature of 1300 C. The sintering operation required about 5 minutes. The block thus obtained has a remanence Br of 2550 Gauss and a field strength of disappearance I C of 1750 Oersteds. The (BI'I)rnax value in the principal direction was 113x10 A block obtained in the same manner but not subjected to an orientation by the first magnetic field treatment had a (BH)max value of 08x10. In both cases the apparent density was 5.0.

Example 3 In the manner described in Example 1, pre-fired powder was stirred with an 8% solution of polyvinylacetate in acetone to form a suspension, use being made of 2 cc. of

the binder solution per gram of the powder. The suspension was introduced into a molding die and arranged in a comparatively weak first magnetic field of 2400 Oersteds. While the first field was applied the binder solution was removed. The blocks so obtained were further treated in the manner described in Example 2. The resulting blocks had a remanencei Br in the principal direction of 2720 Gauss and a field strength of dis appearance I C of 1620 Oersteds. A (BH)max value 1.2 was obtained. At right angles to the preferential direction, the remanence was 1610 Gauss.

Example 4 Pre-fired powder obtained in the manner described in Example 1 was stirred with an 8% solution of polyvinylacetate in acetone to form a suspension, use being made of 1 cc. of binder solution per gram of powder. The solution was placed in a molding die positioned in a first magnetic field of 10,000 Oersted and while in this field, the binder solution was removed. The remaining binder solution was removed by heating to a temperature of 500 C. in a second magnetic field of 2000 Oersted. The blocks were then passed at a rate of mms./min. through a furnace, as described in Example 1, having an atmosphere of air at 1280 C. In the principal direction the blocks had a remanence Br of 2875 Gauss, a field strength of disappearance I C of 1800 Oersteds, a B C value of 1550 Oersteds and a (BH)max value of 1.65 X10 The remanence at right angles to the principal direction was 1625 Oersteds, and the field strength of disappearance I C was 2100 Oersteds. A block obtained in the same manner but not subjected to a first magnetic field treatment had a remanence Br of 2150 Gauss, a field strength of disappearance I C of 2000 Oersteds, a B C value of 1450 Oersteds and a (BH)max value of 0.9 10

Example 5 Pre-fired powder obtained in the manner described in Example 1 was stirred with water to form a suspension; 2 cc. of water being used per gram of powder. This suspension was placed in a molding die, subjected to a first magnetic field and molded to form blocks which were fired in the manner described in Example 4. In the principal direction these blocks had a remanence Br of 2870 Gauss, a field strength of disappearance I C of 1610 Oersteds and a (BH)max value of 1.57 10 These blocks had a remanence Br at right angles to the principal direction of 1625 Gauss.

Example 6 Pre-fired powder obtained in the manner described in Example 1 was molded into blocks in the manner described in Example 4. These blocks were heated to 500 C. without the application of a magnetic field, and were then passed at the rate of 20 mms./min., through an electric furnace, having an atmosphere of air at a temperature of 1280 C. The firing period was about 5 minutes in the preferential direction. The blocks so obtained had a remanence Br of 3025 Gauss, a field strength of disappearance I C of 1550 Oersteds and a (BH)ma.x value of 158x10 Example 7 A pre-fired mixture as described in Example 1, was pulverized bygrinding for 48 hours in a ball mill under alcohol. After being dried the powder was formed into sintered blocks in the manner described in Example 4. In the principal direction these blocks had a remanence Br direction of 3100 Gauss, a field strength of disappearance I C of 1650 Oersteds and a (EH-)max value of 1.72 (1 0. Blocks obtained in the same manner butnot subjected to a first magnetic field treatment had a remanence Br of 2250 Gauss, a field strength of disappearance I C of 1850 Oersteds and a (BH)max value of 0.95X10 The apparent density of the sintered blocks was 4.95.

Example '8 A mixture of 29.5 grams of strontium carbonate (as determined by analysis) and 179 grams of R203 (69.4% by weight of Fe) was ground for 15 hours in a ball mill with alcohol. After the alcohol was removed, the powder was dried and then heated for 2 hours at 1000 C. in an electric furnace having an atmosphere of air. After being cooled the material was ground for 4 hours in an agate centrifugal mill with alcohol. This prefired powder was formed into blocks in the manner described in Example 4. Blocks prepared in this manner had, in the principal direction, a remanence B:- of 2640 Gauss, a field strength of disappearance I C of 2825 Oersteds and a (BH)max value was l.63 l0 The apparent density was 4.34. At right angles to the principal direction the blocks had a remanence Br of 1550 Gauss. A block obtained in the same manner, but not subjected to a first field treatment, had a remanence Br of 2020 Gauss and, a field strength of disappearance I C of 3225 Oersteds, a (BH)ma.x value of 0.99 10 and an apparent density of 4.41.

Example 9 Starting with a mixture of 29.5 grams of strontium carbonate (as determined by analysis) and 163 grams of R203 69.4% by weight of Fe), a pre-fired powder was formed into blocks in the manner described in Example 4. The blocks so obtained had, in the principal direction, a remanence Br of 2700 Gauss, a field strength of disappearance I C of 2900 oersteds and a (BH)max value of 1.67 10 The remanence Br at right angles to the preferential direction was 1670 Gauss. The apparent density was 4.75. Blocks obtained in the same manner but not subjected to a first magnetic field treatment had a remanence Br of 2075 Gauss, a field strength of disappearance, Ho of 3225 oersteds, and a (BH)max value of 088x10 and an apparent density of 4.69.

While we have described our invention in connection with specific examples and specific embodiments, other. modifications thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of this invention as defined in the appended claims.

What we claim is:

1. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition MF12019 in which M is at least. one metal selected from the group consisting of barium, strontium and lead, orienting the crystals of said finely-divided material with preferential directions of magnetization parallel to the principal direction, compacting the so-oriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to 1500 C. to sinter the same into a highly-coherent dense body, and finally magnetizing said body in said principal direction.

2. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition MFe12O19 in which M is at least one metal selected from the group consisting of barium, netically orienting the crystals of said finely-divided material with prfeferential directions of magnetization parallel to the principal direction, compacting the so-oriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to 1500 C. to sinter the same into a highly-coherent dense body,

and finally magnetizing said body in said principal direction.

3. A method of making a permanent'magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition MFerzOm in which M is at least one metal selected from the group consisting of barium, strontium and lead, susstrontium and lead, mag- 1 pending said finely-divided material in a liquid, magnetically orienting the crystals of said finely-divided material with preferential directions of magnetization parallel to the principal direction while the particles are suspended in said liquid, compacting the so-oriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to 1500 C. to sinter the same into a highly-coherent dense body, and finely magnetizing said body in said principal direction.

4. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition MFBraOm in which M is at least one metal selected from the group consisting of barium, strontium and lead, magnetically orienting the crystals of said finely-divided material with preferential directions of magnetization parallel to the principal direction while compacting the sooriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to

1500 C. to sinter the same into a highly-coherent dense body, and finally magnetizing said body in said principal direction.

5. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition MFeizOig in which M is at least one metal selected from the group consisting of barium, strontium and lead, mechanically orienting the crystals of said finely-dividing material with preferential directions of magnetization parallel to the principal direction, compacting the sooriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to 1500 C, to sinter the same into a highly-coherent dense body, and finally magnetizing said body in said principal direction.

6. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition MFeizOrg in Which M is at least one metal selected from the group consisting of barium, strontium and lead, rolling said mass at an elevated temperature to orient the crystals with preferential directions of magnetization parallel to the principal direction, compacting the so-oriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C to 1500 C. to sinter the same into a highly-coherent dense body, and finally magnetizing said body in saidprincipal direction.

7. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition of BaFeieOis, orienting the crystals of said finelydivided material with preferential directions of magnetization parallel to the principal direction, compacting the so-oriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to 1500 C. to sinter the same .into a highlycoherent dense body, and finally magnetizing said body in said principal direction.

8. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition of SrFerzOm, orienting the crystals'of said finely-divided material with preferential directions of magnetization parallel to the principal direction, compacting the sooriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to 1500 C. to sinter the same into a highly-coherent dense body, and finally magnetizing said body in said principal direction.

9. A method of making a permanent magnet magnetically-anisotropic in a principal direction comprising the steps of finely-dividing a mass of material consisting essentially of hexagonal crystals having the composition of PbFemOrs, orienting the crystals of said finely-divided material with preferential directions of magnetization parallel to the principal direction, compacting the sooriented crystals into a body of desired shape, heating the so-compacted body at a temperature of about 900 C. to 1500 C. to sinter the same into a highly-coherent dense body, and finally magnetizing said body in said principal direction.

10. A permanent magnet consisting of a highly-coherent sintered body magnetically-anisotropic in a principal direction and consisting essentially of a compact mass of hexagonal crystals of MFemOis in which M is at least one metal selected from the group consisting of Ba, Sr and Pb, said crystals being oriented with their preferential directions of magnetization parallel to said principal direction.

11. A permanent magnet consisting of a highly-coherent sintered body magnetically anisotropic in a principal direction and consisting essentially of a compact mass of hexagonal crystals of MFerzOw in which M is at least one metal selected from the group consisting of Ba, Sr and Pb, an atomic fraction of not more than 0.4 of the quantity of one of said metals being replaced by Ca, said crystals being oriented with their preferential directions of magnetization parallel to said principal direction.

12. A permanent magnet consisting of a highly-cm herent sintered body magnetically anisotropic in a principal direction and consisting essentially of a compact mass of hexagonal crystals of BaFeizOm, said crystals being oriented with their preferential directions of magnetization parallel to said principal direction.

13. An permanent magnet consisting of a highly-coherent. sintered body magnetically anisotropic in a principal direction and consisting essentially of a compact mass of hexagonal crystals of SrFerzOrg, said crystals being oriented with .their preferential directions of magnetization parallel to said principal direction.

'14. A permanent magnet consisting of a highly-coherent sintered body magnetically anisotropic in a principaldirection and consisting essentially of a compact mass of hexagonal crystals of PbFemOm, said crystals being oriented with their preferential directions of magnetization parallel to said principal direction,

15. A permanent magnet consisting of a highly-coherent sintered body magnetically-anisotropic in a principal direction and consisting essentially of a compact mass of hexagonal crystals of MFeizOw in which M is at least one metal selected from the group consisting of Ba, -Sr,.and Pb, said crystals being oriented with their preferential directions of magnetization parallel to said principal direction, said magnet having in said direction a(B1'I)max of at least 1.l 10 and at least of that of an isotropic magnet of the same composition.

References Cited in the file of this patent UNITED STATE$ PATENTS 1,991,143 Ehlers Feb. 12, 1935 1,997,193 Kato et al Apr. 9, 1935 2,188,091 Baermann Jan. 23, 1940 2,463,413 Neel i Mar. 1, 1949 Guillaud Nov. 27, 1951 

1. A METHOD OF MAKING A PERMANENT MAGNET MAGNETICALLY-ANISOTROPIC IN A PRINCIPAL DIRECTION COMPRISING THE STEPS OF FINELY-DIVIDING A MASS OF MATERIAL CONSISTING ESSENTIALLY OF HEXAGONAL CRYSTALS HAVING THE COMPOSITION MFE12O19 IN WHICH M IS AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF BARIUM, STRONTIUM AND LEAD, ORIENTING THE CRYSTALS OF SAID FINELY-DIVIDED MATERIAL WITH PREFERENTIAL DIRECTIONS OF MAGNETIZATION PARALLEL TO THE PRINCIPAL DIRECTION, COMPACTING THE SO-ORIENTED CRYSTALS INTO A BODY OF DESIRED SHAPE, HEATING THE SO-COMPACTED BODY AT A TEMPERATURE OF ABOUT 900* C. TO 1500* C. TO SINTER THE SAME INTO A HIGHLY-COHERENT DENSE BODY, AND FINALLY MAGNETIZING SAID BODY IN SAID PRINCIPAL DIRECTION. 